NOVEMBER,1916. MONTHLY WEATHER REVIEW. 6% from the hi her illuminated air masses and that diffrac- only on our side of the latter. If the former, than the tion can in Buence only the color of the light. . clouds intercept the purple-colored solar rays; if the The color and the time of occurrence of the western latter, then the purple doe8 not develop where the cloud’s purple light and of the afterglow are in agreement with shadow falls. The former appears to me much the more all three classes of explanpt,ions. The relahe rarhy of probable case. the afterglow and its varying intensity with the osit.ion The western purple light, then, seems to be the evening of the sun speak in favor of the-iden of mirror re ection. glow of the hi her air directly illumined by the setting It seems to me, however, that if the .oause were mirror sun, reflected 8;own into the earth’s shadow to our eyes. reflection from the sunset zone tslie l!ght woul? appear The eastern afterglow shows us mountains and air in- somewhat less scattered and would gve more llght wid directly illuminated by the reflection of this lofty evening shade efkct in the mountains. As a matter of fact I have glow. .,. often wondered, as I stood in the mountains in the after- 7, at glow itself, the shadowless or ver weakly shadowing TWILIGHT PHENOMENA IN ABIZONA, SEPTEYBEB TO character of this eculiar flesh-red ligi t: and how it often DECEMBEB, 1916. a pears not mereP y as the western purple and tho eastern aP te low but fills the whole interveniii space wit.11 By Prof. ANDREWELLICOTT DOIJ~LASS. its a-pervading mysterious, gloomy ye1B ow-red. The [Dated: Deparlniciit of Physim 3nd Astrouomy, University of -4Iizm8, Tucson, breadth of the light-giving surface nnd of t.lie mirror Dec. 2, 1916.1 together are after all an inadsquatmeexplanation, it seems [In his letter transmitting this gper Profemor Douglaee correcta his to me, for this diffuse behavior of the after low. Just as statement, quoted by nie in the EVIEW for August, 1916, 44:434, to t.he effect that red sunseta were not observed by him after the middle the sk is blue and tho mountains and vaf leys are fillrd of September, 1916. His accompanying description of twilight rolora wit8hblue haze durin the dsv, so nom- the sky is yurplc, is closely in accord with the claeaical description b von Besold, which, eo per-oolornd, or re dish yetlow and the mountains and t,ogether with Exner’s classification of the twilidt phenomenon will d be found translated on pages 620 to 63of this number of the REVIEW. vaPI eys aro filled wit.li these colors. If reflectioii throws Therefore. with the consent of Professor Douglaes, I have inserted in the western purple light. into our eyes and upon the east,ern brackets in his text the designation given bv Exner to the hase of mountains, t’hen t,here must. he n much stronger pohbriza- the t.wilight.deecribed. Thus, [(C) Fimt twilight arch] ie to %e inter- tion of the purple. greted aa nieaning that the phase of twili8ht under consideration will e found under (c) of Exner’s classification to have the deeignation If the sky’s blue were a true fluorescence of bhe air, given.--If. H. h’hball.] then the western purple light and the afterglow- would he here blue instead of red. I conclude, from their colors, that Prom September 16, 1916, to tihe present time the the sky’s blue is mere1 a pseudo-fluorescence. writer has wat,ched ever. evening aft,er sunset for the The peculiar form oB a “ salmon-colored spot ” in which occurrence of bright. t,wil ‘ght and afterglow colors, and the western urple light often begins, has not yet been shndow phenomena. The clearness of our atmosphere explained, wR ile the arched form of the western purple permits all t,hese henomena to be seen with greatest seems to be a matter of course.. Evidently we have not. ease down to our x orizon of mountains, which averages vet the last word in esplanation of the western purple about. one degree in elevation in all directions. From bght and of the afterglow. In the case of the afterglow west-northwest to wst,-southwest the average height is we may have to do mt,h the conibined effects of several about ISo, while in the northwest the apparent horizon factors. Nature is. indeed. always more coniplicated descends a very slight amount below the true horizon. than we like to kqsurne in thought,. On! t.lling is quite In the enst t.hc niounta.ins rips from 1’ to 3’ above the certain, thhe afterp.lOw on the ea&m m.oimtuana miid the cast- true horizon. ern 8k.v iS not a direct evening red. but an.inc7abect PrerLing On account3of t.1~clearness of the air t,he sun is never red which k brought down into the ea.rth’s .Q?t.m70ii7 hy mir- under any circumstances faint enough t.o be looked at wriri or difuse rqflcrtion. rc.nQ (1.ifkctiin.n. Put niorc directly hy t,he naked eye. Clouds are rare and two levels sim &: The afterglow in t,he east is the reflcct,eds >lendor of clouds still more rare, so that it heconies possible to of t e western urple light. We may t,hcrrfore cescrihe1 estimate cloiid heights lq- t,he t.inie when direct. sunlight the Rafterglow a o as an indirect, dpen low. Here also is c-enses to illuniinat,c thein. It thus becomes possible here repeat.ed R relation similar to t,hat. of t.5 ie fiist dpe~~glow:to usc nieniis of inrestigat.ion that, would seem incredible The west furnishes light and color, t.he east is illuniinat,cd to an observer nccust,omed only to A4t,ln~it.iccoast. con- thereby. Thus it. conies about t.hat light, nncl color are d i t,ions. stronger in the west than they are siniult.aneouslv in t,he Out, of 80 niglit.s, cloucls have interferred to tl serious east. If one stands on t.he eastern mountitins and wholly extent on only li nights. Plight. cloudiness has prevailed in the afterglow: there appcura no repet,ition of sunset in on over 51) per cent of t.he nights. In fact, it is a con- the west; one sees only a reat surface of purple light. dit,ion of n fcw thin, scat,t.eredciriiis clouds that is most spread over almost the whof; e westmeinsky. favorable t.o t,he heautiful ray phenomena so often Here and there a sheaf of divergent rays [crepuscultlr noticcd. [See Ahlso’s translations of Heim’s and Bezold’s raw] from a mountain or a cloud in the west.ern sky, descriptions of t,liesc crepuscular rap: this REVIEW,p. 622 reitches high up into the recently enlpurpled heavens. and p. 625.1 (See Heini’s fi . 18.) The shadow rays [or crepusaular As soon as the disk of t’he sun is behind the western rays] a pear bf ue-green in t.lie purple. Hence one niaq‘ mountain, a heavy bronze area [(b) Twilight low] is conclua e that the air tmhatmappears to he already in the apparent. estending to a dist,a.nceof 4’ or 5’ in a5 direc- western purple, as we stand down below it, still receives tions from the sui1 itself. At, first this bronze is of a direct rays from the sun at high levels above us. The &her yellowish color, which soon changes to a reddish western purple light therefore arises first from direct il- tone, lasting some 20 minutes after the disappearance lumination of the u per air lapis. Diffusereflection and of t,he sun. diffraction are but tK e processes t.hat conduct the splendor -4fter the sun has really set the clear western sky in down to our eyes in t.he eart,h’s shadow or across to the that. general vicinity shows a structure which seem8 momt aim. likely t,o be due to a high layer of haze in the atmos- This does not enable one to decide. however, whether phere. This structure usually ap ars as a faint, soft the purple light originates in the west behind the cloud or etching of large numbers of paralelp“ lines. In general

Unauthenticated | Downloaded 09/26/21 05:20 AM UTC MONTHLY WEATHER REVIEW. NOVEMBER,1916 these lines have a slight inclination from the horizontal, out to get the elevation of the layer or curtain on which the northern ends bemg a little lower than the southern and shadows are projected. The process is ends. The lines vary in size from a quarter of a degree simple,these 'Y or the length of time after sunset gives a rou h in width and several degrees in length up t,o 1' by 10' or idea of the hundreds of miles of distance between t% e Kear the horizon the size is a.lways small, observer and the clouds which are casting the shadows. growingeven lary arger at the higher e1evat.ions. The coiispicous- This distance is easily shown to be 300 to 400 miles, ness is very different on different nights and the view is for some of the best raylike shadows appear 30 minutes alwap improved by good field glasses. A number of after sunset, at ,which moment the sun is setting at a observations have been made on the motion of this point some 400 miles west of ur. By carrying out this detail, which in every case has been in a soizt,herly or calculation it is eas to show that we am seeing shadows southwesterly direction, and a very rough analysis of cast upon a layer a out. 12 miles above thc surface of the the rate has given something in the neighborhood of earth. 60 miles per Iiour. This structural effect., of course, As u rule, these shadows appear raylike in form, with exists in what otherwise appears to be an absolutely the rays divorging from the sui1 (below the horizon). But clear sky. It rises about %'hi h in the west. and may occasionally rounded or elongated blue cloud shadows are be traced through an aziniut,h o GOo or 55'. Tho pass- observed, with direct sunlight, very deep red in color, ing of the colored sunlight acrossP this structural detail, which has passed underneath the cloud itself, and is lower and lower t,owards the west.ern mountains, was one projected higher in the western sky than the light which of the first phenomena noted last '4 teniber. passes over the cloud. Tho color of these cloud shadows Five minutoes after sunset^ the luo arch of eai-th IS usudly blue but occasionally is a very striking green. shadow [(a) Dark se ment] begins to rise above the [See Abbe's translation of Bezolil, p. 623.1 eastern horizon with a arilliant pink curve [((I) First anti- When the pink afterglow [(d) First purple light] has twilight arch] of sunlit atmosphere immediately above it. sunk low in the west a faint iiik glow suffuses the entire If the night is favorable tho pink arch is cut up into ii horizon [(ej Second anti-twi right]. As this second after- number of alteiiiating blue and >ink rays [see glow Icaves the east it niny be seen as a faint pink glow kx e's translation of Heiin's descri tion, t\ lis issue of t,he [(g) Second purple light] in the mesteni sky at about an REVIEW,p. (5251 all pointing the spot opposite the hour after sunset. sun. Twenty-thma blue shadow rays have been counted The most exquisitely beautiful combination of colors at one time. Some of them may oome from clouds visibln is obtained when there seems to be a long horizontal cloud above the western horizon and the blue shadow may be shadow [(e) First twili lit arch], cutting through [above] traced com letel across the sky froni western cloud to the western afterglow kc) First bright, segment] at about eastern arcl. &en the shadow comes from clouds below 25 minutes after sunset.. Uiider such conditions the the western horizon; but at this oarly nioniont in the lemon- ellow area [(e) First bright se ent] extends evening, it can not usually be traced noaror the westorn from tK e horizon perhaps So in height tffp rough a great horizo6'than 30'. part of the western sky, then comes a nearly com lete As the Dink arch in the east [(a,)First, anti-tu-ilight arch] interruption of shadow [(c) First t,wilight arch] a ove rises hig]r;er and higher above-the horizon, rsnr:hing ai; which is the most superb dis lay of a brilliant red bandE elevation of So,or IO', or ewii 12' before it boc.:oines too [(d)First purple light] extengng u wards perhaps 10' or faint (on one occasion it seemed to appoar at 18' eleva- more and divided into glowing Krepuscular] rays all tion), tho eastorn [anticrepuscular] rays est8euilmore and pointing t.oward the sun, long since out of sight. The mor6 toward the west, bccoining fainter tmd Iiiiiiter and color of t.his ure red glow [(a) First Fyrple light] is ver rarely showing directly overhead. As the pink nrch almost monocR romatic and seems much ihe the color of in t e east really disap ears, 20 to 35 minutes after sunsct, romineiices observed through the C line of hydrogen. a pinkE glow begins to Porni in a groat area estendiiig from ft is so different in the splendor of it,s beauty hom the 10' or 15O up to 60' above the western horizon [(d)Fiist ink edging to the afterglow seen oil clear nights that it purple light]. On perfectly char nights this glow grad- Ras become evident that special conditions are required ually settles toward the horizoii [(c) Fiist twilight arch] to produce it at it.s best. And when seen at its best it retaining for a long timo between it nnd the horizon ti forms one of t.he most. reni:irkable nieteorological dis- deep lemon-yellow color [(c) First bright segment]. A~cJu~;plays ever witnessed by the writer. 30 minutes sftcr siinsot the lenion-yellow has set, [End ... . ! I of civil twilight] and for the iiext 10, 15, or even 20 inin- .- ' utes the pink glow is grn.dually disitppcariiig below the TWILIGHT COLOBS AT MOUNT WILSON, CAL., AUGUST- western mountains. SEPTEMBEB, 1918. When there aro a few cirrus c:louils at the pro or distance to the west of us to cause shadows, the ~Es- By WENDELLP. HOOK, AssiRtant Astronomer. appearance of tho pink arch in the east [(ai First anti- [Dated: Mount Wllson SohObservntorp, Cal., Nov. 1, ISlO.] twili ht arch] with its [aiiticrepuscidar] rays is inime- diate y followed or even accompanied by the formation of Mr. Ford A. Carpenter, Meteorologist, Weather Bureau, Loa Angel-, f cs5 ., tranamita the following description of twili ht colors observed et a splendid series of bright [crepuscular] ra 011 the the Mount Wilaon observatory, contributed by dr. W. P. Hoge.] western sky. These are much brighter than tr osc in the east and at htreach an elevation of 40' or more over In reference to red sunsets my "log book" contains the place of sunset and a length of 60' or SO", if low, the following note under date of August 4, 1916: "Very along the northwestern or southwestern horizon. The brilliant sunsets for several evenings." This indicates base of these ra r at first ma9 bq15" or 10' above the that t.hese brilliant dis lays be an about Au . Their horizon, but as tr ie half hour a ter wisct passes the rays onset was rather suurPden anf they have Yts owly dimin- extend from tho visible horizon in a deep red color u 1 to ished in bri htness, although they were quite marked loo or even 20' in altitude. As many as 22 of t iese during all oP August. The effect is still very noticeable bright rays, separated by ns many blue shadows, have [November 11, particularly in the morning [when] I have been counted at one time. A few of the Tor numerous a good sky line from 25 to 100 miles distant. The dis- messurements of elevation have boen roug rlly worked play is gradually growing fainter.

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